1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus which can improve precision in positioning of a magnetic head.
2. Description of the Related Art
In a conventional hard disk drive of a sector servo type, data tracks are concentrically arranged and used to record data on a magnetic disk as a recording medium. A data track is called a cylinder. Each cylinder has a plurality of sectors. Each sector has a servo area and a data area. In each servo area, servo data is recorded. In each data area, data to be read/written by a user is recorded. The data is referred to as host data. Servo data includes servo detection data for detecting a servo area, address code data for identifying a cylinder, and burst data for positioning a magnetic head to the center of the cylinder. Note that servo detection data has a signal pattern which does not exist in host data to be read/written from/in a data area by a user. As such a signal pattern, data with no inverse magnetization, i.e., continuous erase data, is recorded. In addition, burst data includes four bursts A, B, C, and D.
Burst data is recorded at a predetermined repetitive frequency to prevent a peak shift caused by waveform interference between adjacent reproduced signals or the like. Note that burst data recorded at a low frequency is easier to read. With a decrease in recording frequency, however, a burst pattern area in which burst data is recorded is increased, resulting in a decrease in data area. For this reason, when the minimum frequency of recording frequencies used to record host data is 1 f, the recording frequency for burst data is generally set to be 8/3 f. Note that f corresponds to 1 MHz to several 10 MHz.
In a conventional hard disk drive, data recorded on a magnetic disk is read out as an analog reproduced signal by a magnetic head and is amplified by a head amplifier. The amplified signal is further amplified by an AGC (automatic gain control) amplifier with a gain set by an AGC circuit. Since the amplified reproduced signal contains high-frequency noise, a low pass filter is used to remove the noise. Note that the cutoff frequency of this low pass filter is determined by the characteristics of a magnetic head and a magnetic disk to be used and the S/N (signal/noise) ratio of the overall hard disk drive.
In a hard disk drive, since data recorded in a data area has a wide range of frequency components, a low pass filter to which such data are input preferably has uniform group delay characteristics in its pass band. Note that the group delay characteristics correspond to phase characteristics, and indicate periods from the input of the low pass filter till the output of the low pass filter in each frequency component. In the low pass filter used in the hard disk drive, the group delay characteristics represent a delay amount of the peak position of a reproduced signal between the input and output of the low pass filter. The group delay characteristics are represented by a group delay time.
When the group delay characteristics of the low pass filter greatly vary in accordance with recording frequencies, data having a wide range of frequency components tends to cause a peak shift. More specifically, in burst data recorded at a predetermined repetitive frequency, magnetization inverse occurs at predetermined intervals, and the group delay time corresponding to the frequency is constant. Therefore, the peak positions of reproduced signals uniformly change (delay), and the peak positions of adjacent reproduced signals do not come close to each other. However, as described above, since data recorded in a data area has a wide range of frequency components, magnetization inverse does not occur at predetermined intervals, and the group delay time changes depending on recording frequencies. For this reason, the peak positions of reproduced signals variously change (delay) in accordance with recording frequencies. As a result, the peak positions of adjacent reproduced signals may come close to each other, resulting in a peak shift. For the reasons described above, a low pass filter is required to have uniform group delay characteristics.
With regard to burst data, the amplitude of a reproduced signal needs to be accurately detected. If, however, high-frequency noise is contained in the reproduced signal, since its amplitude cannot be accurately detected, a positional shift from the center of a cylinder cannot be accurately obtained. Therefore, it is difficult to accurately position the magnetic head to the center of the cylinder.
Under the circumstances, in a hard disk drive of a sector servo type, it is demanded that precision in positioning of a magnetic head be improved by accurately detecting the amplitude of burst data while keeping constant with the delay of each peak position of data recorded in a data area.